1. Field of the Invention
This invention relates to a DC power converter (DC power supply), and more particularly to a resonant power converter which is capable of performing zero voltage switching (ZVS) with reduced noise.
2. Description of the Prior Art
In general, a 5 to 10 volt DC power converter is required to operate the semiconductor integrated circuits used in electrical and electronic equipment such as personal computers and AV equipment. In the prior art, as for such DC power converters, power converters using a switching regulator are employed. Such DC power converter using the switching regulator can be formed into a compact size rather than other types of DC power converters using series regulators and shunt regulators and the like. Further, such DC power converters have advantages in that power loss is small and energy conversion efficiency is high. Recently, in order to achieve even greater reductions in power loss, resonant switching converters which are capable of zero voltage switching (ZVS) have been proposed and put to practical use.
In this connection. FIG. 16 is a schematic view of the structure of a prior art resonant power converter which is formed into a full bridge type. As shown in the drawing, this resonant power converter 20 is equipped with a D power source E21, a bridge circuit composed of four MOS-FETs 21 to 24, and a transformer Tr21 which is arranged at the output side of such bridge circuit. In this structure, one end of a primary coil (winding) L21 of the transformer Tr21 is connected to the junction between the MOS-FETs 23, 24, and the other end thereof is connected to the junction between of the MOS-FETs 21, 22 via a resonance capacitor C21 and a resonance coil L24.
Further, the both ends of a secondary coil (winding) L22 of the transformer Tr21 are respectively connected to the anodes of diodes D21, D22, and the cathodes of the diodes D21, D22 are connected to a center tap of the secondary coil L22 via a choke coil L23 and a capacitor C22. Further, both ends of the capacitor C22 form output terminals for connection to a load RL.
In this arrangement, the MOS-PETs 21 to 24 are driven by drive pulses having roughly the same resonance frequency as the resonance circuit constituted from the resonance capacitor C21, the resonance coil L24 and the primary coil L21 of the transformer Tr21 so that alternate switching between a state in which the MOS-FETs 21, 24 are turned on and a state in which the MOS-FETs 22, 23 are turned on is carried out to generate a resonance current in the primary coil L21 of the transformer Tr21. With this result, the resonance current induces a current in the secondary coil L22 of the transformer T21, and therefore a regulated output voltage Vo is generated across the ends of the capacitor C22. Further, because the resonance current flows with a substantially sinusoidal waveform, zero voltage switching can be carried out, and this makes it possible to greatly reduce switching loss and generation of noise.
However, in the prior art resonant power converter, since the resonance caused between the resonance capacitor C21 and resonance coil L24 and the primary coil L21 of the transformer Tr21 is utilized, there is a problem in that the energizing current for the transformer Tr21 becomes too large, which results in large energy loss and an increased size of the transformer Tr21. Further, there is another problem in that this arrangement requires to have the center tap in the secondary coil L2 of the transformer Tr21, which results in complicated structure of the transformer Tr21.
In addition, in the prior art resonant power converter, since the diodes D21, D22 are used as the rectifying means, a large voltage drop occurs in these elements. Therefore, when such a power converter is configured into a large current and low voltage type power converter which is aL trend of the current power converters, the energy loss caused in the diodes becomes considerably large, which results in the factors that cause increased heat generation and decreased conversion efficiency of the circuit of the power converter. Namely, the prior art circuit configuration involves a problem which can not be applied to large current and low voltage type power converters.
The present invention has been made in view of the problems in the prior art described above, and therefore an object of the present invention is to provide a resonant power converter which does not need to provide a center tap in a transformer and has high energy conversion efficiency.
Another object of the present invention is to provide a resonant power converter utilizing synchronous rectification which can reduce energy loss in the rectifying means to improve energy conversion efficiency of the power converter.
In order to achieve these objects, the present invention is directed to a resonant power converter. The resonant power converter comprises an input power source provided in the primary side; switching means connected to the input power source: a resonant transformer having a primary coil and a second coil; and a resonance capacitor which is provided in the secondary aide and is connected in parallel to the secondary coil of the resonant transformer so that series resonance occurs it a current flowing through the secondary side of the power converter.
In the resonant power converter described above, because a resonance current flows between the resonance capacitor arranged on the secondary side of the resonant transformer and the leakage inductances of the resonant transformer, a resonance current will not flow through the primary coil of the transformer, and as a result, it is possible to reduce the excitation current of the transformer.
Further, because zero voltage switching can be carried out, it becomes possible to reduce power loss. Furthermore, because the switching timing of the primary side is not synchronized with the switching timing of the secondary side, the switching noise generated at the primary side and the switching noise generated at the secondary side are not superposed with each other, and this makes it possible to diffuse noise.
In the present invention, it is preferred that the resonant transformer is constructed from an ideal transformer and a resonant coil arranged in the primary side of the ideal transformer, so that the series resonance occurs between the resonant coil and the resonance capacitor.
Further, it is also preferred that the resonant transformer has a leakage inductance, and the series resonance of the current occurs between the leakage inductance and the resonance capacitor.
In the present invention, it is also preferred that the resonant power converter further comprises a current doubler circuit for making the current flowing through the secondary side double, said current doubler circuit being provided in the secondary side and coupled to the both ends of the resonance capacitor. In this case, it is preferred that the current doubler circuit includes a pair of circuits each having a rectifying means and an inductance, in which these circuits are connected in parallel with each other so that a junction between the rectifying means and inductance of one circuit and a junction between the rectifying means and the inductances are respectively connected to the both ends of the secondary coil of the transformer.
In the present invention, it is particularly preferred that the resonant power converter further comprises means for performing synchronous rectification for the current flowing the secondary side, said means including switching elements for synchronous rectification provided in the secondary side so as to be driven by drive signals responsive to the voltage generated across the resonance capacitor and having a sinusoidal waveform. In this case, it is preferred that said switching means includes a pair of switching elements, and these switching elements are driven by the drive signals.
According to this arrangement, since the voltage signal in the form of sinusoidal wave generated across the resonance capacitor is used to drive the switching elements for synchronous rectification to rectify the current flowing through the secondary side of the circuit, a voltage drop can be drastically reduced as compared with the conventional circuit using diodes and thereby it is possible to improve the energy conversion efficiency of the power converter remarkably. Further, there is no need to provide a separate drive circuit for driving the switching elements, the circuit configuration can be made simple and therefore manufacturing cost can be lowered.
In this arrangement, it is preferred that the resonant power converter further comprises a pair of auxiliary coils which are connected to the both ends of the secondary coil, respectively, and the drive signals are obtained from the opposite ends of the respective auxiliary coils which are not connected to the secondary coil. This makes it possible to amplify the drive signals generated at the resonance capacitor.
Further, it is also possible to further comprises waveform processing means for producing pulse waves by processing and shaping the voltage drive signals to be supplied to the switching elements. In this waveform processing means, the voltage drive signals in the form of sinusoidal wave are compared with a predetermined reference value to produce the plus signals.
Moreover, it is also preferred that the resonant transformer is composed of an ideal transformer and a resonant coil connected to the primary coil of the transformer, so that the series resonance occurs between the resonant coil and the resonance capacitor.
In the arrangements described above, a MOS-FET is preferably used as each of the switching elements.
These and other objects, structure and advantages of the present invention will be apparent from the following description of the preferred embodiments in conjunction with the appended drawings.